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arXiv 1108.0604astro-ph.GA
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马头PDR中的H2CO：尘埃颗粒冰帽的光解吸

H2CO in the Horsehead PDR: Photo-desorption of dust grain ice mantles

V. Guzmán, J. Pety, J. R. Goicoechea, M. Gerin, E. Roueff

备注：
Accepted to A&A, 9 pages, 6 figures, 5 tables; corrected typos
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摘要：

目的：首次研究了晶粒表面化学在马头光解离区（PDR）中的作用。研究方法：我们在马头星云中对朝向PDR及其相关致密核的几条H2 CO旋转线进行了深入观测，那里的尘埃是冷的（$\Td \simeq 20-30$ K）。我们用218.2 GHz的p-H2 CO $~3_{03}-2_{02}$谱线图（12”角分辨率）补充了这些观测结果。我们确定的H2 CO丰度使用一个详细的辐射转移分析，并比较这些结果与PDR模型，包括纯气相化学或气相和粮食表面化学。结果：PDR和致密核中H-核的H2 CO丰度（$\simeq 2-3 \times 10^{-10}$）相似。在致密的核心纯气相化学模型再现所观察到的H2 CO丰度。因此，表面过程并没有显着贡献的气相H2 CO丰度的核心。相比之下，在PDR中需要在尘埃颗粒表面形成H2 CO并随后光解吸到气相中来解释所观察到的气相H2 CO丰度，因为气相化学本身并不能产生足够的H2 CO。不同的生成路线的归属通过不同的测量的H2 CO的邻-对位比来加强：致密核显示平衡值（$\sim3$），而PDR显示非平衡值（$\sim2$）。结论：H2 CO冰的光脱附是将大量气相H2 CO释放到马头PDR中的有效机制。

英文摘要：

Aims: For the first time we investigate the role of the grain surface chemistry in the Horsehead Photo-dissociation region (PDR). Methods: We performed deep observations of several H2CO rotational lines toward the PDR and its associated dense-core in the Horsehead nebula, where the dust is cold ($\Td \simeq 20-30$ K). We complemented these observations with a map of the p-H2CO $~3_{03}-2_{02}$ line at 218.2 GHz (with 12" angular resolution). We determine the H2CO abundances using a detailed radiative transfer analysis and compare these results with PDR models that include either pure gas-phase chemistry or both gas-phase and grain surface chemistry. Results: The H2CO abundances ($\simeq 2-3 \times 10^{-10}$) with respect to H-nuclei are similar in the PDR and dense-core. In the dense-core the pure gas-phase chemistry model reproduces the observed H2CO abundance. Thus, surface processes do not contribute significantly to the gas-phase H2CO abundance in the core. In contrast, the formation of H2CO on the surface of dust grains and subsequent photo-desorption into the gas-phase are needed in the PDR to explain the observed gas-phase H2CO abundance, because the gas-phase chemistry alone does not produce enough H2CO. The assignments of different formation routes are strengthen by the different measured ortho-to-para ratio of H2CO: the dense-core displays the equilibrium value ($\sim3$) while the PDR displays an out-of-equilibrium value ($\sim2$). Conclusions: Photo-desorption of H2CO ices is an efficient mechanism to release a significant amount of gas-phase H2CO into the Horsehead PDR.

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